I'm a late diagnosed ADHDer too. There is no magic bullet, but the fact you are aware of it, and want to learn, is already a huge step in the right direction. I'm not sure my parents would ever have accepted ADHD as a thing, but their constant criticism and disapproval was a huge factor in eroding what little self-esteem I had. Excessive criticism for not being able to accomplish seemingly straightforward tasks (like tidying your room) is one of the key contributing factors for RSD. I would sit and agonise over my messy room for hours, days. Paralysed by overwhelm. And I still do it now. It's not deliberate or lazy, it's exhausting.
Masking is another huge one to look out for. I spent my teenage years hiding all the things I struggled with - personal organisation, personal hygiene - as we're not motivated to look after ourselves (and my parents were pretty clueless about hygiene, grooming and all the stuff that's important to a teenage girl - so they didn't help). I was able to get away with things like last minute homework as I was academically very able, but I was also told I was lazy for not putting in consistent effort. I present as outwardly confident and articulate, but I'm inwardly totally dependent on external validation and have no resilience.
Try and foster a relationship with lots of open communication - especially around the things she struggles with that aren't deemed socially acceptable. She may be carrying a lot of shame, and hiding it from you. She may tell lies to try and cover up the things she's forgotten to do, hasn't been motivated to do, or feels ashamed about. She might be too young now, but being able to talk about her struggles openly would be a lifeline.
Fatigue may also be a huge factor. I was a really active dancer, but I used to come home and hide under the duvet to rest. My mum says she could never understand why I was tired all the time, and now I think was a combination of holding it all together, and chronic anxiety about things catching up with me. I really struggle with fatigue as an adult, and I think it's all down to anxiety. Adhd for me has meant a life lived in fear. Fear of failure, fear of consequences, and fear of never realising my potential.
With homework, while novelty is appealing to our brains, conversely it takes a monumental effort to engage in something that doesn't interest us. Concentrating on something when our brains are directing us to anywhere else is like having an internal fight. I still have to do that now with client projects that don't personally interest me. Body doubling is a good technique to try, and helps with procrastination. Basically you sit and work side by side. Not on the same thing, but harmoniously. It has an amazing effect on procrastination. We also struggle with all or nothing thinking, so breaking tasks down into small chunks can really help too.
If you can help her find an outlet for her busy brain, it will really help. We're brilliant at creative ideas, problem solving and helping others. What we can't do is execute plans ourselves very well, which can lead to frustration and constantly feeling like we're failing.
I could go on but I think I've said enough! Thank you for asking the question, and good luck.